WEBVTT 00:01.230 --> 00:07.440 It's time to introduce Luthra Harper in the section I'm going to describe Reduce Harper, which actually 00:07.440 --> 00:11.190 is different and a bit complicated than previous a Harper's. 00:12.100 --> 00:17.950 In order to explain how Ridge's helper actually works, I'm going to start with a classical example, 00:17.950 --> 00:23.860 which you may come across if you try to find information about reduce at first, as usually I'm going 00:23.860 --> 00:26.190 to compare this helper to follow up. 00:26.950 --> 00:29.590 Suppose that we have a right with some numbers. 00:31.230 --> 00:35.370 For example, 100, 200 and 300. 00:37.210 --> 00:43.750 Then we want to sum those numbers and return the total sum as a single value for that, I'm going to 00:43.750 --> 00:50.230 create a variable sum, which actually will be initial value, and let's set it as zero. 00:51.630 --> 00:53.370 After that creates for a loop. 00:54.330 --> 01:01.050 White's verbal eye equals zero, then eyes less than no length. 01:03.590 --> 01:12.530 And the increment inside coal prices, right, verbal some because of that, we want to add numbers 01:12.530 --> 01:18.440 and then save value after each iteration we want to use plus equals operator. 01:19.840 --> 01:21.040 And then numbers. 01:24.780 --> 01:27.180 OK, let's run in Castle Deferrable some. 01:30.370 --> 01:35.920 Reload the page and see that we have 600, the total sum of items in a right. 01:36.760 --> 01:37.250 All right. 01:37.270 --> 01:41.320 It works fine, but no, I'm going to achieve the same using reduce. 01:42.220 --> 01:43.930 Let's make you a little border. 01:46.060 --> 01:49.000 And then create a new variable called a total. 01:50.770 --> 01:52.440 Assigned to it numbers. 01:53.370 --> 01:55.750 And use your reduce helper. 01:56.280 --> 02:02.970 Again, we have to pass your anonymous function, this function receives at least two parameters, generally 02:02.970 --> 02:03.510 JavaScript. 02:03.510 --> 02:08.390 The first parameter is called as previous or sometimes as an accumulator. 02:08.910 --> 02:10.170 You can use any of them. 02:10.680 --> 02:14.790 So this parameter actually accumulates values after each iteration. 02:15.720 --> 02:21.150 As for the second parameter, it's again current item as it was in case of previous. 02:21.150 --> 02:22.740 Helper's so right. 02:22.740 --> 02:23.280 No. 02:25.940 --> 02:33.350 Rudi's Harper may have more parameters like index of a right and the right self, but using those parameters 02:33.350 --> 02:34.680 are optional. 02:35.420 --> 02:43.950 Now let's return previous plus no actually reduce Harper itself receives two parameters. 02:44.390 --> 02:49.970 First, one is callback function, which we have already passed here, and second one is the value of 02:49.970 --> 02:51.210 the previous parameter. 02:51.860 --> 02:57.290 In other words, it's an initial value and in this case we need to set it as zero. 02:57.980 --> 03:00.080 Let's log total variable in council. 03:04.280 --> 03:08.740 Reload and we have the same result, which is 600. 03:09.410 --> 03:13.290 All right, as usual, let's see how it is, how it works. 03:14.180 --> 03:16.520 So here we have a right with numbers. 03:16.550 --> 03:24.890 And as you saw, we have set previous parameter or an initial value as zero on the first iteration of 03:24.890 --> 03:27.890 function is executed for the first item 100. 03:28.400 --> 03:32.600 The value of previous parameter is zero, which is added to 100. 03:34.570 --> 03:41.850 After the first iteration, the value of previous parameter becomes 100, then starts second iteration, 03:42.250 --> 03:48.450 the function is executed for the second item 200, which will be added to previous value 100. 03:49.180 --> 03:56.440 So after the second iteration will get three hundred, then start a third iteration, the function is 03:56.440 --> 03:58.680 executed for the third item of array. 03:59.230 --> 04:02.390 It's added to previous value, which is 300. 04:02.680 --> 04:07.080 And finally, as a result, we got 600 as a single value. 04:08.200 --> 04:08.890 Makes sense. 04:10.370 --> 04:10.830 All right. 04:11.380 --> 04:13.390 This is the way how reduced Hopper works. 04:13.750 --> 04:17.020 Let's go back to Brackett's and write some more real world examples. 04:17.950 --> 04:20.770 Suppose that person has fixed monthly salary. 04:20.770 --> 04:27.010 And also besides that, he has weekly bonuses, assumed that we want to get the total monthly income 04:27.010 --> 04:27.680 of that person. 04:28.240 --> 04:29.680 So create an array. 04:31.240 --> 04:38.200 And call it weekly earnings, let's insert in that area some numbers. 04:43.140 --> 04:46.300 Then create another variable and call it income. 04:47.760 --> 04:56.520 We're going to use again to reduce Holper, so let's assign to that variable weekly earnings that reduce. 04:57.910 --> 05:05.440 Then anonymous function, this function will receive two parameters for previous value, let's insert 05:05.440 --> 05:11.050 here previous and as a current item, write weekly earning. 05:14.070 --> 05:21.710 In order to get this some of those earnings, we have to return previous plus weekly earnings. 05:24.020 --> 05:29.580 And as we said, person has fixed monthly salary, which will be added to weekly earnings. 05:29.990 --> 05:32.360 We're going to set it as initial value. 05:32.750 --> 05:41.750 So right as the second parameter for reduce helper, for example, 5000, let's log in council variable 05:41.750 --> 05:42.290 income. 05:46.380 --> 05:46.980 Revolt. 05:49.150 --> 05:55.420 And we have here the total monthly income of person, which consists of weekly earnings and also a fixed 05:55.420 --> 05:55.930 salary. 05:57.540 --> 06:03.660 All right, I believe you guess how useful and convenient reduce Harper should be, but you also may 06:03.660 --> 06:06.970 think that we can use to reduce Harper only with numbers. 06:07.470 --> 06:08.700 Actually, it's not right. 06:09.180 --> 06:12.240 And in order to prove that, I'm going to write another example. 06:13.290 --> 06:17.370 Suppose that we have a race for first names and four last names. 06:17.610 --> 06:19.650 So create variable first names. 06:21.240 --> 06:35.460 And insert here some names, John, then Jane and Mary also create another variable for last names and 06:35.460 --> 06:40.010 place here, the items, Smith, Brown and dope. 06:41.660 --> 06:48.680 And suppose that we want to create one new array, which will include full names of the persons like 06:48.680 --> 06:55.640 John Smith, Jane Brown and Mary Jo, so create a new variable and name it as full names. 06:58.490 --> 06:59.630 Then assigned to it. 07:00.670 --> 07:04.570 First names and use here, reduce helper. 07:06.510 --> 07:08.280 Place, anonymous function. 07:09.250 --> 07:11.200 With two parameters. 07:12.180 --> 07:13.320 Previous and. 07:14.900 --> 07:17.060 Then as current item, right, first name. 07:20.910 --> 07:27.390 So, as we said, full names should be a new arraying, therefore reduce Holper will have as a second 07:27.390 --> 07:32.540 parameter and actually as an initial previous value and empty a right. 07:32.790 --> 07:38.370 And in order to accumulate first names and last names in that array, we're going to use push method. 07:38.970 --> 07:39.270 So. 07:39.270 --> 07:39.550 Right. 07:39.570 --> 07:41.850 Previous that push. 07:43.370 --> 07:51.830 That inside parenthesis, right, first name plus the space, and now we need to get proper last names 07:52.280 --> 07:58.850 for that, we have to use second array last names, but actually it's a right and it doesn't represent 07:59.240 --> 08:01.200 current item like first name. 08:01.640 --> 08:07.880 So in order to get last names, we need to iterate through this array and get the relevant items for 08:07.880 --> 08:08.060 that. 08:08.060 --> 08:09.830 We are going to use little trick. 08:10.310 --> 08:14.330 As we said previously, reduce Harper can receive more than two parameters. 08:14.900 --> 08:20.950 One of them is indexed, which generally should represent the index number of first names array. 08:21.710 --> 08:24.710 But because of that, both arrays have the same length. 08:25.310 --> 08:30.740 Three I'm going to use in this parameter for last names, array items. 08:31.660 --> 08:39.700 But again, remember that generally in this parameter represents in the case of Iraq, for which reduce 08:39.700 --> 08:43.180 Harper is used in this case, the array of first names. 08:44.080 --> 08:44.530 All right. 08:44.860 --> 08:47.400 So let's go ahead and write last names. 08:49.960 --> 08:58.030 And in square brackets index and the last thing to do is to return previous value. 09:00.230 --> 09:03.490 Which actually will give us a final full names. 09:03.530 --> 09:03.860 All right. 09:05.910 --> 09:07.160 OK, let's run it. 09:07.160 --> 09:08.780 Council the variable names. 09:11.220 --> 09:12.150 Reloader page. 09:13.250 --> 09:18.950 And you see that we have a new era which consists of full names, John Smith, James Brown and Mary 09:18.950 --> 09:22.280 Jo, the result that we wanted to get. 09:22.870 --> 09:23.380 All right. 09:23.900 --> 09:29.300 I've written this example just for demonstration that reduced Harper can be used not only with numbers, 09:29.750 --> 09:31.250 but with other data as well. 09:32.290 --> 09:34.980 OK, this was all about release, Harper. 09:35.030 --> 09:39.590 It's a little bit more complicated than previous Harper's, but I hope you could understand it well. 09:40.340 --> 09:40.790 All right. 09:40.940 --> 09:41.590 Let's move on.